1. Field of the Invention:
This invention relates to casing seals for wellheads, and in particular to a metal seal for sealing between the casing and the wellhead bore.
2. Description of the Prior Art:
In a completed oil or gas well, one or more strings of casing is cemented in the well. A wellhead is located at the surface for supporting the upper end of the casing. The wellhead includes a lower portion through which the casing extends. A casing hanger supports the casing in the lower portion of the wellhead.
In one technique, the casing hanger fits around the casing, and the upper end of the casing will be cut off a selected distance above the wellhead after cementing. A casing seal or packoff will be placed between the casing and the bore of an upper portion of the wellhead. This packoff prevents leakage from the annulus between the casing and the wellhead.
In many wells, the produced fluid will be at a fairly warm temperature as it reaches the surface of the casing. The warm temperature can cause the casing to expand axially. The wellhead, however, will not move axially. This results in a slight amount of axial movement of the casing relative to the wellhead.
In the past, elastomeric seals were used as packoffs primarily. These seals would tolerate a slight amount of axial movement of the casing relative to the wellhead. However, metal seals are now preferred for many oil field applications. Metal seals are longer lasting and are not subject to deterioration from certain well fluids to the extent that elastomeric seals may be. However, a metal seal requires a very precise fit in order to accomplish sealing. Also, axial movement would damage the sealing ability of the metal seal.
In U.S. Pat. No. 4,711,832, Charles D. Bridges, Sep. 20, 1988, a metal seal is illustrated for sealing between casing and the wellhead. The metal seal includes two eccentric rings. These rings can be rotated to accommodate slight misalignment of the axis of the casing relative to the axis of the wellhead. The inner sealing ring has an inner face that seals against the exterior of the casing. This inner face contains bands that are axially spaced apart. The bands are soft enough to deform when the seal ring is pressed into contact with the casing.
While this type of arrangement is satisfactory for a static seal, axial movement of the casing relative to the ring can cause problems. It could damage the seal face on the seal bands, destroying the effectiveness of the seal.